1. Field of the Invention
The present invention relates to a method for preparing high-purity p-isobutylstyrene. More specifically, it relates to a method for preparing high-purity p-isobutylstyrene by first subjecting o- and/or m-isobutylethylbenzene, which is a novel substance, to a dispropornation reaction to form p-isobutylethylbenzene, and then dehydrogenating the latter.
p-Isobutylstyrene is useful as an intermediate which can be utilized to inexpensively and economically manufacture .alpha.-(4-isobutylphenyl)propionic acid (trade name Ibuprofen) which is important as a medicine such as an anti-inflammatory drug or the like.
2. Description of the Prior Art
Heretofore, .alpha.-(4-isobutylphenyl)propionic acid has been synthesized by various suggested methods. One of these suggested methods comprises subjecting p-isobutylstyrene to a hydroformylation reaction or a Reppe reaction (Japanese Laid-open Patent Application Nos. 51338/1977, 6233/1977, 97930/1977 and 10545/1984).
This method for preparing .alpha.-(4-isobutylphenyl)propionic acid from p-isobutylstyrene is economically excellent, because p-isobutylstyrene which is a starting material is a simple and stable compound and the hydroformylation reaction or the Reppe reaction does not consume any expensive reagents and the like.
It is known that p-isobutylstyrene is manufactured by hydrogenating p-isobutylacetophenone, followed by dehydrating, as disclosed in Japanese Patent Publication No. 35899/1984. Furthermore, p-isobutylstyrene can be also manufactured by first reacting isobutylbenzene with acetaldehyde in the presence of a sulfuric acid catalyst to form 1,1-bis(p-isobutylphenyl)ethane, and then decomposing the latter in the presence of an acid catalyst, as disclosed in Japanese Laid-open Patent Application No. 24527/1986. In these methods, however, many steps are necessary, a great deal of the catalyst is consumed, and the used waste catalyst is too strong in acidity to easily throw away, which increases the manufacturing cost of p-isobutylstyrene.
In view of the above-mentioned situations, the inventors of the present application, as a result of intensive researches, have developed a method for preparing p-isobutylstyrene by selectively dehydrogenating only the ethyl group of p-isobutylethylbenzene.
However, with regard to the manufacturing methods of p-isobutylethylbenzene, there are merely a very few conventional techniques, and according to one example described in Beilstein, EIV5 (Sys. Nr. 470/H445), 1-(4-ethylphenyl)-2-methylpropane-1-one is reduced with potassium hydroxide and hydrazine in a diethylene glycol solvent to produce p-isobutylethylbenzene. In this method, however, 1-(4-ethylphenyl)-2-methylpropane-1-one which is the raw material is very expensive, and hydrazine which must be used as the reducing agent is very dangerous in treating. They impede the industrialization of this method. Moreover, it is also known that p-isobutylethylbenzene can be produced as a by-product of the catalytic decomposition reaction of 1,1-bis(p-isobutylphenyl)ethane, as disclosed in examples in Japanese Laid-open Patent Application No. 37743/1986, but this method is not preferable, because p-isobutylethylbenzene is only the by-product and thus its productivity is extremely low.
For these reasons, another economical method for the manufacture of p-isobutylstyrene is desired. Thus, the present inventors have further advanced the researches, and as a result, they have developed a method for preparing p-isobutylethylbenzene by the utilization of alkylation. That is, one example of this method comprises ethylating isobutylbenzene. In this case, position selectivity in the ethylation is not usually high, and so the ethylation gives the production of o- and m-position isomers in addition to a p-position isomer. In this case, p-isobutylethylbenzene can be collected by separation, but it is uneconomical that the separated o- and m-isobutylethylbenzenes are thrown away. In addition, useful applications for o- and m-isobutylethylbenzenes have not been known yet.
Here, the present inventors have tried disproportionating o- and m-isobutylethylbenzenes by the use of an acid catalyst to convert them into p-isobutylethylbenzene.
However, when the dispropornation reaction is carried out in the presence of the acid catalyst, the skeleton isomerization of an isobutyl group in isobutylethylbenzene also takes place. It is extremely difficult to separate secondary butylethylbenzene produced by the skeleton isomerization from isobutylethylbenzene which is its isomer, because boiling points of these compounds are close to each other.
From the above-mentioned viewpoint, the present inventors have advanced the researches, and eventually the present invention has now been achieved.